Diesel engine exhaust is a heterogeneous mixture which contains not only gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and nitrogen oxides (“NOx”), but also condensed phase materials (liquids and solids) which constitute the so-called particulates or particulate matter. Often, catalyst compositions and substrates on which the compositions are disposed are provided in diesel engine exhaust systems to convert certain or all of these exhaust components to innocuous components. For example, diesel exhaust systems can contain one or more of a diesel oxidation catalyst, a soot filter, and a catalyst for the reduction of NOx.
Oxidation catalysts that contain platinum group metals are known to facilitate the treatment of diesel engine exhaust by promoting the conversion of both HC and CO gaseous pollutants and some proportion of the particulate matter through oxidation of these pollutants to carbon dioxide and water. Such catalysts have generally been contained in units called diesel oxidation catalysts (DOC's), which are placed in the exhaust of diesel engines to treat the exhaust before it vents to the atmosphere. In addition to the conversions of gaseous HC, CO, and particulate matter, oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) also promote the oxidation of nitric oxide (NO) to NO2.
The total particulate matter emissions of diesel exhaust are comprised of three main components. One component is the solid, dry, solid carbonaceous fraction or soot fraction. This dry carbonaceous matter contributes to the visible soot emissions commonly associated with diesel exhaust. A second component of the particulate matter is the soluble organic fraction (“SOF”). The soluble organic fraction is sometimes referred to as the volatile organic fraction (“VOF”), which terminology will be used herein. The VOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust. It is generally present as condensed liquids at the standard particulate collection temperature of 52° C. in diluted exhaust, as prescribed by a standard measurement test, such as the U.S. Heavy Duty Transient Federal Test Procedure. These liquids arise from two sources: (1) lubricating oil swept from the cylinder walls of the engine each time the pistons go up and down; and (2) unburned or partially burned diesel fuel.
The third component of the particulate matter is the so-called sulfate fraction. The sulfate fraction is formed from small quantities of sulfur components present in the diesel fuel and oil. Small proportions of SO3 are formed during combustion of the diesel fuel, which in turn combines rapidly with water in the exhaust to form sulfuric acid. The sulfuric acid collects as a condensed phase with the particulates as an aerosol, or is adsorbed onto the other particulate components, and thereby adds to the mass of TPM.
One key aftertreatment technology in use for high particulate matter reduction is the diesel particulate filter. There are many known filter structures that are effective in removing particulate matter from diesel exhaust, such as honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal filters, etc. However, ceramic wall flow filters, described below, receive the most attention. These filters are capable of removing over 90% of the particulate material from diesel exhaust. The filter is a physical structure for removing particles from exhaust, and the accumulating particles will increase the back pressure from the filter on the engine. Thus, the accumulating particles have to be continuously or periodically burned out of the filter to maintain an acceptable back pressure. Unfortunately, the carbon soot particles require temperatures in excess of 500° C. to burn under oxygen rich (lean) exhaust conditions when oxygen is used for the carbon oxidation. This temperature is higher than what is typically present in diesel exhaust. However, another mechanism for the soot oxidation is its reaction with NO2, which takes place with sufficient rates of reaction in the temperature interval between 250 and 500° C. The reason for the upper boundary temperature is the thermodynamic equilibrium between NO and NO2 in the presence of oxygen, which results in low NO2 concentrations by increasing temperature.
Active regeneration processes are normally initiated by altering the engine management to raise temperatures in front of the filter up to 500 to 630° C. for an oxygen based soot oxidation and up to 300 to 500° C. for an NO2 based soot oxidation. Depending on driving mode, high exotherms can occur inside the filter when the cooling during regeneration is not sufficient (low speed/low load or idle driving mode). Such exotherms may exceed 800° C. or more within the filter. One common way that has been developed to accomplish active regeneration is the introduction of a combustible material (e.g., diesel fuel) into the exhaust and burning it across a flow-through diesel oxidation catalyst (DOC) mounted up-stream of the filter. The exotherm from this auxiliary combustion provides the sensible heat (e.g. about 300-700° C.) needed to burn soot from the filter in an acceptable period of time (e.g. about 2-120 minutes).
Provisions are generally introduced to lower the soot burning temperature in order to provide for passive regeneration of the filter. The presence of a catalyst promotes soot combustion, thereby regenerating the filters at temperatures accessible within the diesel engine's exhaust under realistic duty cycles. In this way a catalyzed soot filter (CSF) or catalyzed diesel particulate filter (CDPF) is effective in providing for >80% particulate matter reduction along with passive burning of the accumulating soot, and thereby promoting filter regeneration.
Future emissions standards adopted throughout the world will also address NOx reductions from diesel exhaust. A proven NOx abatement technology applied to stationary sources with lean exhaust conditions is Selective Catalytic Reduction (SCR). In this process, NOx is reduced with ammonia (NH3) to nitrogen (N2) over a catalyst typically composed of base metals. The technology is capable of NOx reduction greater than 90%, and thus it represents one of the best approaches for achieving aggressive NOx reduction goals. SCR is under development for mobile applications, with urea (typically present in an aqueous solution) as the source of ammonia. SCR provides efficient conversions of NOx as long as the exhaust temperature is within the active temperature range of the catalyst, the operating window.
New emission regulations for diesel engines around the world are forcing the use of more advanced emission controls systems. These systems will need to reduce both total particulate matter and NOx by about 95 percent. The engine manufacturers have multiple emission system options to meet the new regulations but one option is the combination of an active filter system for particulate reduction and a selective catalytic reduction system.
One system configuration that has been proposed in the literature involves a diesel oxidation catalyst (DOC) positioned downstream from the engine, a catalyzed soot filter (CSF) positioned downstream from the DOC, a reductant injection system position downstream from the CSF, a selective catalytic reduction (SCR) catalyst positioned downstream from the reductant injection system, and an optional ammonia oxidation (AMOX) catalyst positioned downstream from the SCR catalyst. The system also typically includes a hydrocarbon injection system located downstream from the engine and upstream from the DOC.
This system configuration offers several advantages for the overall system functionality. Having the DOC in the first position allows it to be placed as close as possible to the engine ensuring rapid heat up for cold start HC and CO emissions and the maximum DOC inlet temperature for active filter regeneration. The CSF being in front of the SCR will prevent particulate, oil ash and other undesirable materials from being deposited on the SCR catalyst thus improving its durability and performance. Having platinum group metal oxidation catalysts in front of the SCR allows for an increase in the NO2 to NO (or NO2 to NOx ratio entering the SCR which is known to increase the reaction rate of the NOx reduction occurring in the SCR if properly controlled. An example of such a system is described in U.S. Pat. No. 7,264,785.
This system configuration, however, also is problematic because the DOC often comprises platinum group metals (PGM) dispersed on a refractory metal oxide support. Due to the large amounts of PGM used, these catalysts are relatively expensive. Additionally, in fuel with high sulfur content, such as the fuel in developing and emerging countries, the sulfur reacts to form SO3, which acts a poison to the DOC. The activity of the DOC is, thus, negatively impacted, and filter regeneration cannot be sustained in sufficient forms.
Accordingly, there is an ongoing need to investigate and provide alternative system strategies to improve the treatment of exhaust gas streams containing NOx and particulate matter, especially for fuels containing high sulfur concentrations.